It is well recognized that polyisocyanates which are liquid at room temperature have obvious processing advantages over solid polyisocyanates; see for instance U.S. Pat. No. 4,115,429. Polysicoyanates such as the commercially available and widely used toluene diisocyanate (TDI) and 1,6-hexamethylene diisocyanate are potentially harmful due to their relatively high vapor pressure and accordingly, certain safety precautions are employed to minimize harmful physiological effects. For this reason, various attempts have been made, either to start with diisocyanates that are normally liquid at room temperature and to reduce their physiological effects by certain procedures or to start with diisocyanates that are solid at room temperature and to convert these into liquid form. In both cases, however, one usually obtains either isocyanates of higher valency, i.e., tri- or higher polyisocyanates or higher molecular weight diisocyanates or a combination of these effects.
Important diisocyanates which are solid at room temperature and which are readily available on a large commercial scale are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof which melt at 39.degree. C. and 34.5.degree. C., respectively. Attempts have already been made to liquefy both the 4,4'-diphenylmethane diisocyanate and a mixture of the 4,4'-diphenylmethane diisocyanate and a small amount of the 2,4'-isomer. Thus, for example in U.S. Pat. No. 3,644,457, 1 mol of a diphenylmethane diisocyanate is reacted with from about 0.1 to about 0.3 mol of poly-1,2-propylene ether glycol. While the products made according to this patent have met with commercial success, they still suffer from a serious drawback. Specifically, it has been found that these adducts generally will crystallize anywhere from 5.degree. C. to as high as 25.degree. C. In fact, when supplied in commercial quantities, these adducts are generally transported in heated trucks. Additionally, in order to thaw the materials it is generally necessary to heat them to somewhere in excess of 50.degree.to 60.degree. C. While in warmer climates there may not be any problem, in colder areas where the product may be stored in tanks over a period of time, this tendency to crystallize can become a very serious problem. Similar attempts to form liquid diphenylmethane diisocyanates have been described, for example, in U.S. Pat. Nos. 3,384,653 and 3,394,164. The attempts to liquefy in both of these instances were based on the addition of, in one case, a trihydrocarbyl phosphate, and, in the other case, small amount of phosphoric acid. In any event, the storage stability of both of these types of products is again quite good around room temperature, but as the temperature decreases, both types of materials tend to crystallize.
Isocyanato-terminated prepolymers of about 600 to about 4500 molecular weight, e.g., prepared from the reaction of an excess of 4,4'-diphenylmethane diisocyanate (MDI) with (i) linear or slightly branched polyesterdiols with/without polyestertriols (reaction products of lower aliphatic glycols with/without small amounts of glycerol and alkanedicarboxylic acids), or (ii) dipropylene glycol, comprise products generally having melting points of about 10.degree. C. to about 30.degree. C., and higher, depending primarily on the amount and structure of the polyol employed. Quasi-prepolymers containing, by weight, from about 50% to about 60% of the polyol segment (e.g., a polyol from adipic acid, ethylene glycol, 1,4-butandiol) are generally liquid products at room temperature. When the MDI content is increased to about 60%, or more, the resulting quasi-prepolymers are oftentimes crystalline products in the 15.degree.-25.degree. C. range. Quasi-prepolymers comprised of MDI and about 10-15% of the di- and/or tripropylene glycol(s) are oftentimes crystalline at about 15.degree. C. -20.degree. C. In this state, the quasi-prepolymers are more difficult to process, exhibit greater tendency towards dimerization and trimerization reactions, and detract from optimal physical characteristics of the end polyurethane product.